The present invention relates to a system for detecting the start timing of a shift after a shift command has been outputted in an automatic transmission for a vehicle.
As is well known in the art, the automatic transmission of a vehicle has its shift accompanied by a change in the rotation of an engine or other rotary elements. Thus, the inertial torque is absorbed by frictional engagement means to damp the shifting shock. In order to reduce the heat to be absorbed by the frictional engagement means thereby to damp the shifting shock and improve the durability of the frictional engagement means, moreover, the control to be executed in the prior art is to lower the engine torque during the shifting operation.
Since the control for the shift such as the control of lowering the engine torque during the shifting operation is started after the start of a shift in the automatic transmission, it has to be premised by an accurate detection of the start of the shift.
The shifting operation of the automatic transmission is accompanied by a fluctuation of the output torque and a change in the revolution speed (i.e., R.P.M.). Generally in the prior art, therefore, the start of a shift is decided if the difference between the product of the output R.P.M. and the gear ratio and the input R.P.M. exceeds a predetermined reference value. However, these changes in the R.P.M. are caused when an inertia phase is entered. In the aforementioned deciding method, therefore, the start of the shift is decided from the instant when a delay occurs from the actual start of the shift. Thus, there arises a disadvantage that the shift cannot be controlled highly accurately.
Since a torsional vibration of a power transmission line due to a torque fluctuation occurs in a torque phase preceding the inertia phase, there is proposed in the prior art a system for deciding the start of a shift by noting the occurrence of the torsional vibration, as in Japanese Patent Laid-Open No. 54/1992. In this system, the R.P.M. of a predetermined rotary element is predicted on the basis of the hysteresis of the change in the rotation so that the start of a shift is decided from the deviation between the predicted value and the actual R.P.M.
The aforementioned system, as disclosed in Japanese Patent Laid-Open No. 54/1992, detects the start of a shift in terms of the rotational fluctuation in the torque phase preceding the inertia phase so that it can eliminate the delay of the detection of the shift timing to some extent. Since, however, the aforementioned predicted R.P.M. of the rotary element providing a reference for deciding the start of the shift contains causes for disturbances such as the vibration of the vehicle or the torque fluctuation, the decision of the start of the shift has its accuracy deteriorated if it is based upon the prediction. If the detection of the hysteresis of the change in the R.P.M. of the rotary element is elongated to eliminate such disadvantage, a delay is caused in the decision of the start of the shift. Thus in the aforementioned system of the prior art, the control of lowering the engine torque may be delayed.